(i) Field of the Invention
The present invention relates mainly to wood-cased color pencil leads and color pencil leads for mechanical pencils. More specifically, it relates to non-baked color pencil leads which are remarkably excellent in mechanical properties such as bending strength and pointed end strength and which have a smooth writing performance. Furthermore, it relates to color pencil leads and a method for preparing the same by which the conventional color pencil leads not erasable with an eraser or the color pencil leads erasable with the eraser can be optionally prepared and which permits noticeably shortening a drying time and decreasing a manufacturing cost.
(ii) Description of the Prior Art
Conventional non-baked color pencil leads can be prepared by a wet process which comprises, if necessary, dissolving and swelling an organic polymeric material as a binder in a solvent such as water and/or a plasticizer, kneading the same with a wax, a filler, a pigment and the like, extruding the kneaded material, and then drying the extruded articles to remove the solvent and/or the plasticizer therefrom.
In this wet process, however, it is necessary that a drying temperature is set to the melting point or less of the wax so as to prevent the bleeding of the wax, and therefore a tremendous period of time is taken in the drying step. Moreover, leads prepared by a dry process, in which the solvent and/or the plasticizer is not used and the drying step is omitted, are also put on the market, but the quality of these leads is inferior to that of the leads obtained by the wet process.
In connection with the strength and the writing performance of the conventional color pencil leads obtained by the wet process, the adhesion of the filler and pigment by the binder is partially impeded with the wax, so that the pencil leads are worn at the drawing and exert lubricating properties and a staining power.
However, when the wax is kneaded with a blend composition containing the filler, the pigment, the binder and the like, followed by molding, the effects of the smooth writing performance and the excellent staining power are remarkable, but the deterioration of strength is also large. In consequence, any satisfactory color pencil leads balanced between the strength and the writing performance have not been obtained so far.
Heretofore, it has been difficult to erase, by an eraser, lines drawn on a paper with a usual color pencil.
On the other hand, the non-baked color pencil leads which are said to be erasable with an eraser are also put on the market, and most of these leads contain the binder, which is a non-polar organic material, and the wax as the main components in order to be easily removed from the fibers of a paper. In addition, the staining power is increased by softening the leads so as to permit drawing under weak pencil pressure, whereby the lead composition is prevented from getting into between the fibers of the paper at the drawing to obtain erasability.
However, the pigment tenaciously sticks between the fibers of the paper at the depiction by the wax, and the lead composition which has once stuck between the fibers of the paper cannot be removed completely by the adsorbing power of the eraser. Thus, this erasability is not practically satisfactory.
The removal of the drawn lines by the adsorbing power of the eraser can be achieved by using the oil, the fat and/or the wax which is liquid at ordinary temperature for all of the wax.
However, in the conventional wet process, the drying step is effected by heating at 50.degree.-60.degree. C., and therefore the oil, the fat and/or the wax bleeds and the shape of the leads cannot be maintained any more in the drying step. In consequence, the conventional wet process cannot practically utilize such an oil, fat and/or wax.
Thus, it can be conceived that color pencil porous leads are first prepared, and they are then impregnated with the oil, the fat and/or the wax which is liquid at ordinary temperature.
In order to form the open pores in the pencil porous leads, some techniques have been heretofore suggested which are a technique of adding a sublimable material, a technique of adding a pyrolysis material and a technique such as an acid and an alkali solution treatment. However, in every technique, an organic pigment is heated to a level higher than the thermal deterioration temperature of this organic pigment, and so the color pencil porous leads having a sharp color cannot be obtained.
The baked color pencil leads are only put on the market which can be prepared by kneading a clay, a white filler, a binder and the like, molding the kneaded material, baking the molded articles in an oxygen atmosphere to form baked white pencil porous leads, and then impregnating them with an ink. With regard to these baked color pencil leads, their erasability with the eraser is substantially satisfactory, but since it is impossible to impregnate the porous leads with a highly concentrated pigment ink, the impregnation is carried out by the use of a dye ink. The leads obtained by using the dye ink have the problem that they are very poor in light resistance and staining power as compared with the non-baked pencil color leads in which the pigment is used.
In order to overcome these drawbacks, a novel preparation process of color pencil leads has been contrived by a part of the inventors of the present invention, and this novel process comprises blending a water/oil repellent substance with other materials, forming open pores owing to the less wettability of the water/oil repellent substance, and then impregnating the resultant porous articles with an oil, a fat and/or a wax.
Examples of the optimum white water/oil repellent substance having excellent lubricating properties which can be used in the preparation method of the novel color pencil leads include two of graphite fluoride and boron nitride. However, in order to uniformly disperse 2 to 20 parts by weight of the water/oil repellent substance, sufficient care must be taken, since this substance is originally less wettable. In addition, the water/oil repellent substance is used in a relatively small amount, and therefore it can be presumed that some segregation is present.
When the water/oil repellent substance segregates, the open pores formed owing to the less wettability of the water/oil repellent substance also segregate, and this fact further gives rise to the segregation of the oil, the fat and/or the wax with which the porous articles are impregnated. At the time of drawing, consequently, the color pencil leads have a tendency to wear down in a nonuniform state, so that the coloring of drawn lines is uneven. In view of the fact that the more uniform lines are desired, the segregation of the water/oil repellent substance is not preferable.